A High-Redshift Arrival-Time Bound from the 13 GeV Photon of GRB 080916C

GRB 080916C, at redshift 4.35, is among the most distant bursts with GeV photons, and its most energetic early photon, about 13 GeV, arrived 16.5 s after the trigger. Charging that entire interval to propagation — an assumption this paper flags as very conservative, since the burst's high-energy emission itself turned on late — bounds the linear dispersion scale at 1.538e+18 GeV; charging only the offset from the high-energy component's onset raises the figure to 5.074e+18 GeV. A Planck-scale linear term would have produced a delay of only 2.1 s over this path — well inside the observed offset — so this burst does not by itself exclude Planck-scale linear dispersion; its bound is set by its clock, not its distance, along a line of sight whose lookback time is about 12.4 billion years. The framework examined here predicts zero propagation delay; the observation is consistent with that prediction. Computed from published data; not a new measurement.